In the last few years there has been an explosive growth in the demands placed on existing communication systems to provide rapid transmission of voice, data, and video signals. These demands place a premium on the provision of dependable, inexpensive, broad band transmission means for use in such systems. Presently, the most widely used transmission means are cables, fiber optic waveguides, and wireless connections. However, users choosing between these various transmission media must balance their need for high capacity data links against the high cost of such links.
Of the transmission media available, cables are typically the least expensive, and are used as data links in most privately owned computer networks. However, the rate of data transmission on cables is limited by the capacitance of the cables. As the rate of data transmission is increased, the capacitance of the cable shunts increasing portions of the signal, resulting in signal attenuation and distortion. In addition, impedance mismatches in cables produce reflections of high frequency signals that interfere with and degrade primary signals. Furthermore, dielectric losses force the designer to confront a maximum cable length that can be used without amplification. For these reasons, cables are generally limited to transmitting electromagnetic signals at frequencies below 1 GHz. Very low loss cables, such as Heliax, which are capable of transmitting higher frequency signals, are expensive to produce.
The medium of choice for communication systems in most large scale operations is the fiber optic waveguide. These waveguides transmit data encoded as optical signals, and provide bandwidths superior to electromagnetic cable systems. However, fiber optic waveguides are expensive to manufacture, install, and maintain, and these costs are not likely to fall in the foreseeable future. As a result, fiber optic waveguides are used primarily in toll communication systems where the high costs are supported by access fees paid by the large numbers of users. For example, wide area networks (WANs) for transmitting video, voice, and data signals over long distances support sufficient traffic to justify the capital outlay necessary for fiber optic systems. In contrast, local area networks (LANs), which are typically used for communications among the computers within a company, are privately owned and supported. Consequently, the high cost of fiber optics is difficult to justify, particularly since these systems are not easily moved if the company changes location.
High frequency wireless communications using microwaves are limited by the number of channels available for broadcast type transmission, and the capital cost and logistical problems of implementing broad based, directional transmission signals.